How do I mark an invoice paid if the customer wants to pay cash instead of using his card on file?

Hello there, gutter.

I'll ensure you can mark the invoice as paid even if the customer pays through cash in QuickBooks Online (QBO).

You can manually mark the invoice as paid and record it through Receive Payments.

Here's how:

1. Click on the +New button, then select the Receive Payment option.
2. From the Customer dropdown, choose the name of the customer.
3. Choose Cash in the Payment method dropdown.
4. From the Deposit dropdown, select the account you put the payment into. 
5. Click the checkbox for the invoice you're recording the payment for in the Outstanding Transactions.
6. You can enter a Reference number and Memo if needed.
7. Then, click Save and Close to mark the invoice as paid.

Additionally, you can check out this article for more details on handling invoice payments, especially if you want to record partial payments in QuickBooks: Record invoice payments in QuickBooks Online.

Furthermore, I'm attaching this link for future reference if you want to personalize your sales transactions in QBO: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Please keep me posted if you have follow-up questions about managing invoices and their payments in QuickBooks. I'm always around to help.

@guttersupplyjen- 

If you don't deposit the cash into one of your bank accounts in QB, then you'll need to create a bank account called "Cash on Hand" or "Petty Cash" and use that as the 'Deposit to' account when you receive payment.